Documents EP 0 233 120 and US 2002/031164 disclose methods of measuring the thickness of a layer of material by using light beam emitter means that emit a light beam so as to heat said layer of material.
The measurement methods described in those two above-mentioned documents present the disadvantage of enabling the thickness of the layer of material to be determined in relative manner only.
With methods of that type, it is necessary to have recourse to a reference sample having a layer of known thickness in order to measure the thickness of a layer of material on any sample.
In addition to being generally of low accuracy, such methods are often complex to perform.
Other methods enable the thickness of the layer of material to be measured in a manner that is absolute, in the sense that they do not require any reference sample.
Thus, the document “Photothermal radiometry for spatial mapping of spectral and materials properties” (Nordal and Kandstad, Scanned Image Microscopy, Academic Press London, 1980) discloses photothermal radiometry techniques for measuring the thickness of a layer of material.
Those photothermal radiometry techniques are based on measuring variations in the surface temperature of the layer of material by using an infrared detector that measures the heat flux radiated by the layer of material that is heated locally at a heating point by an intense light beam emitted by a powerful light source, e.g. a laser source.
The heat flux radiated by the layer of material is generally of low intensity and very noisy, such that the signal-to-noise ratio of the measurement is poor.
It is also known to use a synchronous detection method in which a light beam is used with optical power that is modulated sinusoidally at a known modulation frequency.
Modulating the optical power of the light beam leads to the temperature of the layer of material to be heated by the light beam emitted by that light source also being modulated.
The measurement signal that is delivered by the photothermal detector, which signal is representative of the heat flux radiated by the layer of material from the localized heating point, then also presents oscillations due to the temperature variations at the surface of the layer of material.
In a synchronous detection method, use is also made of a phase locked amplifier (or “lock-in amplifier”) for performing analog filtering.
The amplifier accepts as input a signal that is not filtered, which signal is the product of said measurement signal delivered by the photothermal detector multiplied by an “optical” signal representative of the modulated optical power of the light beam, and at its output it delivers a signal representative of the phase shift between the sinusoidal component of the measurement signal at the modulation frequency of the light beam and the optical signal. By way of example, such an amplifier is used in Document U.S. Pat. No. 4,513,384.
It is then possible to calculate the thickness of the layer of material from the value of this phase shift (see for example “Thermal-wave detection and thin-film thickness measurements with laser beam deflection”, Applied Optics, Vol. 22, No. 20, pp. 3169-3176, Oct. 15, 1983), and from a set of thermo-physical characteristics of the layer of material, for example its conductivity and its thermal diffusivity, its optical absorption coefficient for the light beam, or indeed its thermal resistance.
Nevertheless, the use of synchronous detection with a phase-locked amplifier requires measurements to be performed over a measurement time that is long in order to average out the non-filtered signal so as to improve the signal-to-noise ratio.
Also known is document “Phase lock-in laser active pyrometry for surface layer characterization of tokamaks walls” (Melyukov et al., 10th International Conference on Quantitative Infrared Thermography, Jul. 27-30, 2010, Quebec), which discloses a method of measuring the thickness of a layer of material, by:                measuring the modulated optical power by means of a photodiode;        measuring the radiated heat flux by means of a pyrometer; and        acting in real time, by means of synchronous detection and a phase locked amplifier, to determine the phase shift between the measurement signal and the optical signal on the basis of the two preceding measurements.        
Such a measurement method, which makes use of two distinct detectors, is unfortunately of poor accuracy for determining the phase shift between the sinusoidal component of the radiated heat flux and the modulated optical power.
Furthermore, the measurement times of such a method are long.
Finally, it is poorly adapted to making measurements while “hot” on a layer of material radiating a large amount of heat flux that would run the risk of damaging the laser and the detector, e.g. a layer of material presenting a temperature that is greater than or equal to 150 degrees Celsius (° C.).